Move juicy anchoring bits into a plugin of Annotator

Threading events fix and tests

Rather than looping through all threads, use the references property
to find the parent directly. It should be guaranteed to exist since
one cannot reply to something which does not have an id.

Furthermore, when there is no parent reference, the root serves as
the parent. Without considering this, deleting top level annotations
leave stubs in the UI.

Add some tests that the events do what they're supposed to. NB: the
thread method itself should probably be tested.

Import Annotator

Rather than using prebuilt JavaScript in the vendor directory, build the
relevant Annotator plugins from CoffeeScript imported in the previous
commit.

We don't use I18N or provide locale files, so this serves no purpose.

Rather than using a prebuilt vendored Annotator from our fork, this
commit imports the entire forked Annotator source into the h repository.
Our intent is not to maintain this fork within h, but to remove the fork
repository and incrementally move back to dependence on upstream
Annotator.

Introduce Store resource

This makes it explicit that the function is both a constructor and an
angular resource.

Previously, the text between the markup when the math button was pressed in the markdown editor read $$LaTeX or MathML$$. We don't support MathML, so I've changed it to $$Insert LaTeX$$.

Remove the canAnnotate flag

Refactor WebSocket origin security

Fix MathJax not loading in Chrome extension.

Rather than using the cross site request forgery token in the URL
for the WebSocket, check the HTTP Origin header. All spec-compliant
user agents send a proper Origin header so this is sufficient to
protect users from malicious cross-site access to the WebSocket.

As a consequence, the front-end code to bootstrap the streamer can
be simplified. The streamer no longer has any provider. Its URL and
transport are passed explicitly to the ``open`` method.

While I was here, I added support for the ``protocols`` argument to
the ``open`` method, added support for the ``onopen`` and ``onclose``
handlers, set the client identifier on the ``$http`` service default
headers, aligned the state constants with the standard ones, and
ensured that the socket cannot be closed twice.

Fix small KaTeX display math.

KaTeX default renders math in an inline style. This was creating problems for KaTeX rendered display, and making it looked bunched up and small.